The technique of transferring a layer from an original substrate to a temporary or final target substrate is increasingly used in microelectronics. This technique has many applications, of which only two will be mentioned here by way of illustrative and non-limiting example. For example, it is used to produce SQI (silicon on insulator) substrates used in particular to produce fast components with low power consumption. It is also used to produce composite substrates that limit costs by avoiding the use of costly bulk substrates. This is the case with bulk silicon carbide substrates, for example.
One prior art method of transferring a thin layer from a source substrate to a target substrate is described in French Patent Application No. FR 2 681 472 (also U.S. Pat. No. 5,374,564 to M. Bruel) and its various improvements (hereby incorporated by reference). This patent discloses a process comprising the following steps in particular:                creation by ionic implantation of a buried weakened region within the source substrate delimiting within that substrate the thin layer to be transferred,        assembling the source substrate to the target substrate at the free surface of the thin layer, and        applying thermal and/or mechanical energy to cause a fracture in the source substrate in the weakened region.        
A problem can arise if heat treatment is required to induce some or all of the fracture in the weakened region and the source and target substrates feature materials with very different coefficients of thermal expansion. This is the situation, for example, if it is required to transfer a film of silicon onto a fused silica substrate. Heat treatment can induce high internal stresses within the structure formed by assembling the two substrates, by virtue of the difference in their coefficients of thermal expansion, and these high internal stresses may damage the structure. These stresses can also cause damage at the moment of fracture proper, since at this time the structures immediately relax when they are suddenly dissociated. There is therefore at this moment a sudden jump in the stresses in each structure, i.e. the structure formed of the transferred thin layer attached to the target substrate and the structure formed by the remainder of the source substrate. If its magnitude is too high, this jump can damage at least one of these two structures.
To solve this problem, it would be necessary, at the fracture temperature, to be able to monitor precisely the stresses within the structure formed by assembling the two substrates, in order to maintain them below an acceptable stress level or even to minimize them.
More generally, the problem is that of controlling the stresses within a heterostructure (i.e. a complex structure made by assembling at least two different materials) at the moment of dissociation of the heterostructure when that dissociation necessitates a change of temperature.